Common Misconceptions in Aerodynamics: Part 2

Testing is an Absolute Necessity, Not an Unnecessary Frivolity
The claim: Since we know what the “ideal” shape looks like and can predict airflow over a car, all we have to do is build something to that shape and it will be guaranteed to work. There’s no need to test anything.
Eliminating guesswork by...more guessing. Sounds legit.

The reality: Because of the complexity of the flow around a car, “automobile aerodynamics is still dominated by empiricism,” as Hucho wrote in Aerodynamics of Road Vehicles. Simulations have replaced some of the development previously done in wind tunnels as computers have become more powerful and faster, but there is still no way (and likely never will be) to design a low-drag car by following simple principles of shaping such as the “template” discussed previously, despite aerodynamicists searching for one for the past half century. In real life, it simply doesn’t work; the flow around cars is too complex. Yet scads of people frequenting online forums spend a great deal of time and effort building aerodynamic devices or structures on their cars based on the idea that a little theory is enough to go on. Due to fear of discovering the devices don’t work as they intended or lack of knowledge of any reliable test techniques or both, they rarely if ever bother to verify that these devices do what they claim or they use techniques that are not reliable or sensitive enough to indicate a change in aerodynamic force with any confidence.
Like this.

I happen to think now that it’s mostly people afraid of being wrong, in addition to simply not knowing any better because they have been fed a steady stream of simplified “template” BS everywhere they’ve looked for advice online (this was certainly my experience). Julian Edgar, the Australian author, journalist, and former editor of Autospeed, inadvertently provided an example of this when he joined a well-known web forum before publishing a new book on aerodynamic testing for amateur modifiers. Edgar codified some existing testing techniques and developed groundbreaking new ones and shared them on the forum; almost immediately, a cadre of posters, led by one in particular who was seen as some sort of aerodynamic guru (and the developer of a two-dimensional “template” based on the Klemperer/Jaray model), jumped down his throat. They simply could not believe that actual, on-road testing of aerodynamic changes to a car could be more reliable than predictions based on the “template.” The vitriol with which they defended their approach—which is essentially one of guessing and hoping—betrayed their dogmatism. Belief in the “template” was just that; one poster even used that word to describe his support of it.
That’s a pity, because the community at this well-known web forum is made up of just the sort of people who would benefit from testing new aerodynamic devices to reduce drag, would likely learn a lot from it, would be much more credible with measured and documented improvements that they could share, and could pass a lot of this knowledge on to others since they have a tendency to evangelize. Yet, visit the forum today and you will still see most of the posters content to sit and speculate, to compare production and concept cars to the “template” (based on the center profile cautioned against by Barnard and others), and to devote energy and resources to constructing aerodynamic add-ons based solely on it. One of them has even dressed up these profile comparisons, calling them “dimensional analysis”—which in this case is just a fancy way of saying “looking at the shapes” (don’t confuse this with actual dimensional analysis, which is a method in fluid mechanics “for reducing the number and complexity of experimental variables that affect a given physical phenomenon,” as explained by Frank White in Fluid Mechanics, 2017). The problem with this pseudo-“dimensional analysis” is it doesn’t tell you anything useful about what the air does around that shape (especially if you’re looking only at a center profile); the only way to do that is by measurement, whether in a wind tunnel or a computer simulation or on the road.

Above: measuring things. Here, I was recording pressures on the side of my car with and without ducts ahead of the wheels.

And now, thanks mostly to the efforts of Edgar, we have a lot of ways to measure things on the road. Useful techniques such as wool tuft testing for flow visualization and pressure measurement on body panels already existed, but Edgar also came up with an entirely new method for measuring changes in drag directly: throttle-stop testing. Since a car will reach its top speed when the power required to overcome mechanical and aerodynamic drag matches the engine’s output, he reasoned, artificially limiting engine power temporarily should allow the measurement of drag changes at legal speeds on the road. Simply place a stop on the throttle to fix its travel, make an aerodynamic change to the body, and compare the speeds reached; if the speed goes up, drag has been reduced; if it goes down, drag has been increased. The method is simple, reliable, and cheap.
This has been anathema to the armchair aerodynamicists on the web, however. Edgar’s thread on throttle-stop testing received 8,700 views and garnered more than a hundred replies—yet only three people reported trying it out. Even fewer attempted measuring body panel pressures on the road. More than a decade of unchallenged “template” theory had poisoned the waters, generating a cult-like belief in the practice of “guess and hope” rather than test and measure. The fact that testing and measurement is exactly what every textbook on vehicle aerodynamics prescribes is apparently lost on all these hobbyists. Scibor-Rylski commented on the importance of testing: “Our accumulated experience and know-how of car aerodynamics is gathered from many scale models tested in wind tunnels and from investigations on full-sized vehicles carrying measuring equipment which are tested on the road under natural conditions.” What a shame that a huge part of the hobbyist community interested in aerodynamics won’t do it simply because they don’t believe it necessary, even as they idolize shapes that were produced through testing.
I understand that testing can seem daunting. It also seems like a lot of effort—because it is! It takes a great deal of thought and planning, time finding a suitable testing location and waiting for good weather, energy to make temporary mockups and figure out how you’re going to test them, and consternation interpreting the results. I get it. But the rewards far outweigh the effort, and in the end, testing is far more satisfying than shooting from the hip and hoping your guesses are working, never knowing if they are or not. Who on earth wants to live like that? There’s no need to premise aerodynamic modifications to your car on a belief that they work when you can, for the cost of some cheap supplies and a little bit of time, know that they work.

Initial testing of fins, made quickly out of cheap corrugated plastic and taped in place. Tuft testing didn't tell me anything useful, so I'll test them again using a different method such as pressure measurement.

Kent Kelly and Harry Holcombe, two GM stylists, wrote in a 1963 paper, “Technical apathy is often a far greater deterrent to the acquisition and application of vehicle aerodynamic data than any excessive expenditures of time or money.” That is certainly still true today.


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